Four decades ago, the Soviet Union developed the powerful nerve agent known as “Novichok,” or “Newcomer.” This chemical superweapon, far more powerful than anything in the U.S. arsenal, remained a secret until after the fall of the U.S.S.R.

Fortunately, only a small amount of this toxin was ever created. But unfortunately — very unfortunately — a dose turned up in Salisbury, England, where it was used in an apparent March 4 assassination attempt against Sergei Skripal, a former Russian spy who had been recruited by British intelligence years earlier. Also affected were Skripal’s daughter, Yulia Skripal, and one of the first-responders on the scene, Detective Sergeant Nick Bailey.